Electro-mechanical resonators



April 28, 1970 Filed April 19, 1967 D. W. THORN ET AL ELECTRO-MECHANICAL RESONATORS 2 Sheets-Sheet 1 & :&m M M MM Aw ATTORNEYS United States Patent 3,509,387 ELECTRO-MECHANICAL RESONATORS Denis William Thorn, Great Baddow, Keith Henry Littlebury, Witham, and Eric Davies, Danbury, England, assignors to The Marconi Company Limited, London, England, a British company Filed Apr. 19, 1967, Ser. No. 632,088 Claims priority, application Great Britain, Dec. 19, 1966,

Int. Cl. H01u 7/00 U.S. Cl. 310-83 Claims ABSTRACT OF THE DISCLOSURE This invention relates to electro-mechanical resonators and more specifically to electro-mechanical resonators of the kind in which an electrically driven mechanically resonant beam or bar is employed as a frequency determining element in an electrical circuit such as an oscillator or filter. Such resonators are extensively used in tone signalling equipment utilising the voice channel spectrum in a telecommunication system. The resonant beam is generally electro-magnetically or piezo-electrical- 1y driven and, in present common practice, is fitted with a make and break contact connected to open and close an associated electrical circuit as the beam vibrates resonantly. The frequency of vibration in such cases is usually somewhere in the range of 100 to 3000 c./s. and, because the best commercially available contacts have a distinctly limited useful life even at very low current rating (somewhere about 3 X 10 operations) the need for obtaining a long operational life (in terms of time) has led to thetone signalling systems using such resonators being confined to those in which only bursts of tone frequency are employed. This is a most undesirable practical limitation since it precludes using a monitor tone to establish continuity of the communication link. Considerable advantages would follow the adoption of continuous tone instead of bursts of tone but this is not satisfactorily practical with a resonator which operates contacts by its vibration. The present invention accordingly seeks to provide improved electro-mechanical resonator devices in which make and break contacts are eliminated.

According to this invention an electro-mechanical resonator comprises a vibratory beam of pre-determined resonant vibration frequency, a U member of substantial stiffness relative to that of the beam and across the limb ends of which said beam is bridged and to which said beam is attached at its ends, electro-ma'gnetic or piezo-electric means for causing said beam to vibrate resonantly, and a piezo-electric device adapted to provide electrical output at the resonant vibration frequency when the beam vibrates said device being mounted on or deposited on the beam-U member structure at a location at which vibration occurs when the beam is vibrating and having an electrode deposited on or otherwise bonded to said structure at said location.

The piezo-electric device providing the electrical output may be mounted or deposited on any of a number 3,509,387 Patented Apr. 28, 1970 of different positions on the structure. One suitable posit1on is on the beam near one end thereof. Another suitable position is on the U member at a location which is a vibration node: e.g. if (the preferred case) the U member is a rectangular U consisting of two parallel limbs and a cross piece at right angles to the same, near one end of the cross piece.

If the beam is to be piezo-electrically driven this may be achieved by a second piezo-electric device also mounted or deposited on the beam-U structure and having an electrode deposited or otherwise bonded to the structure at a location at which vibration occurs when the beam is vibrating. This second device may also be mounted or deposited at any of a variety of different positions e.g. near one end of the beam or it may be mounted or deposited on the U member at a location which is not a vibration node. Thus, in the case of a U member of rectangular shape the second device may be mounted or deposited on the U member near one end of the cross piece of the U.

If the beam is to be electro-magnetically driven this may be achieved by an electro-magnet upstanding from the middle of the U member and extending towards the middle of the beam with its free end spaced from said beam.

In one embodiment of the invention the beam has electrical input and output pieZo-electric devices mounted or deposited thereon, each with an electrode bonded or deposited thereon, one being over one end of the beam the other being over the other end of said beam.

In another embodiment of the invention the beam is electro-magnetically driven by an electro-magnet upstanding from the middle of the U member and having its free end adjacent but spaced from the beam and an electrical output piezo-electrical device is mounted or deposited on said U member near one end of the part thereof which joins the limbs thereof.

Any of the embodiments of this invention may of course be arranged to operate as a self-maintaining resonant oscillator by amplifying output from the electrical output means thereof and feeding the amplified output into the driving means for the beam.

Resonators in accordance with this invention may conveniently be mounted by mounting means carrying the U member at vibration nodes thereon. In the case of a rectangular U member such nodes occur on the limbs of the U. In such a case the resonator may be conveniently carried by means of brackets attached to the U member at these nodes.

The invention is illustrated in the accompanying drawings in which FIGURE 1 illustrates diagrammatically a preferred form of beam-U member structures: FIGURES 2 to 5 show various embodiments diagrammatically; and and FIGURE 6 illustrates diagrammatically one way in the beam-U member structure can be carried by means of brackets attached to the U member at vibration noles. Like references denote like parts in all the figures.

Referring to FIGURE 1 the beam-U member structure therein shown comprises a resonant vibratory beam 1 of metal or other suitable material which is bridged across and attached at its ends to the ends of the limbs 2 of a rectangular U member also having a cross piece 3 which is integral with the limbs. The beam, limbs and cross piece are of rectangular cross section. The U member constrains the beam 1 and i of substantially greater stiffness than that of the beam.

In the embodiment shown in FIGURE 2 two piezoelectric crystals 4, 5 one serving as an electrical input or driving means and the other as an electrical output means, are mounted near the ends of the beam, extending over those ends and the adjacent ends of the limbs of the U member. In the case illustrated 4 is the driving crystal, 6 being the electrical input terminals and 7 the electrical output terminals. Each crystal has an electrode firmly bonded to e.g. deposited on, the beam where it is located. The bonding is represented by the thick lines 4A and A. When the driving crystal is energized at the frequency of resonance of the beam and, of course, suitably polarised, the said beam vibrates as conventionally indicated in broken lines in FIGURE 2. If A.C. output from the output crystal (also suitably polarised) is amplified and fed in to the input crystal, the device becomes a self maintaining oscillator and the beam will vibrate continuously at its resonant frequency. Such an oscillator is shown in FIGURE 3 in which 8 is the amplifier.

If two resonators as illustrated in FIGURE 2, are located at opposite ends of and are connected by a communication channel they may be subjected to different ambient temperatures and, when the operating Q values are high (around 1000) care will usually have to be taken to ensure that the difference, due to temperature differences, between their resonant frequencies shall not be too much to be acceptable. In many cases satisfaction of this requirement will involve mounting the resonators in thermostatically controlled housings.

The amplitude of beam vibration is related to the A.C. input amplitude applied to the driving crystal, the properties of the piezo-electric device, the dimensions of the beam and the material of which it is made. If a multiplicity of frequencies is applied to the driving crystal the device will act as a filter and provide a selected output frequency determined by the resonant frequency of the beam. It will be seen that the device, like other embodiments of this invention, has no contacts so that it may be used satisfactorily in signalling systems using continuous tones. Moreover the power requirements as respects the driving signal are lowconsiderably lower than those of a resonator which has to operate make and break contactsthe Q value is high, and the frequency stability is good.

If a device as illustrated by FIGURE 2 is used as an oscillator at one end of a communication channel to supply electrical oscillations over that channel to a frequency selective resonator, also as illustrated'by FIG- URE 2, at the other end of the channel, then, as already stated, difiiculties may arise if the temperature to which the two devices are subjected are materially different since different temperatures will produce different resonant frequencies. This is due to the liability for the existence of substantial temperature coefficients caused by possible variations in coefficients of expansion between the beam, the crystals and the bonding agents but more particularly caused by large changes in Youngs modulus of the crystals with temperature in crystals as at present commercially available. The use of thermostatically controlled housings for the devices will, at least to a large extent, solve these difficulties but such housings are costly. The difficulties can be reduced, but not eliminated, by reducing the areas of contact between the beam and the crystals. This expedient is, however, obviously severely limited as to its applicability, since it results in reduction of transducer efiiciency. FIGURE 4 shows a very simple and effective way of reducing these difficulties.

In FIGURE 4 the driving crystal is dispensed with and instead the beam is driven electro-magnetically by an electro-magnet 9 having a winding 10 and upstanding from the middle of the cross piece 3 towards the middle of the beam 1 from which its free end is suitably spaced. The magnet is fed from input terminals 6A at the appropriate resonant frequency. The magnet is suitably polarised either electrically or magnetically i.e. either by superimposing -D.C. on the A.C. input or by making the core inside the winding 10 a permanent magnet. The output piezo-electric crystal 5 is mounted on the U member at other than a vibration node-as shown it is mounted near one end of the cross-piece 3 where it detects vibration due to the beam vibration. The vibration movement at this location of the crystal is relatively low compared with that at the ends of the beam but adequate movement can be obtained by increasing the power input to the electro-magnet. From the viewpoint of temperaturefrequency coefiicient, the embodiments of FIGURE 4 has considerable advantages over that of FIGURE 2 since the electro-magnetic drive is mechanically divorced from the beam and the crystal 5 is so loosely coupled to the beam that its effect on resonant frequency with temperature variation is quite smallnormally insignificant. With an arrangement as shown in FIGURE 4 a frequency-temperature stability determined substantially only by the beam material is readily achievable. Known materials of substantially zero temperature coefiicient and stable values of Youngs modulus with respect to temperature are available and may be used with advantage for the beam and U member structure.

FIGURE 5 shows a modification of FIGURE 4 in which another crystal 11 is mounted, more or less symmetrically with the crystal 5, at the other end of the cross piece 3. The crystal 11 may be used to provide a second output or the device illustrated by FIGURE 5 may be used as a resonant filter of low temperature/frequency coefficient but large insertion loss.

The arrangement of FIGURE 4 has the advantage that it will only respond at the fundamental frequency of the beam but of course the said beam will respond at this frequency if the magnet is energised at a suitable multiple or sub-multiple of the fundamental.

Nodal points exist at points N on the limbs of the U member and any of the embodiments illustrated can therefore be conveniently mounted by carrier brackets or the like attached at these points. Such carrier brackets are exemplified at B in FIGURE 6.

Although in the particular embodiments described, the piezo-electric devices are constituted by piezo-electric crystals, the present invention is not limited, of course, to the use of such crystals. Instead of using piezo-electric crystals in carrying out this invention, any or all of the piezo-electric devices may be formed by a vapour deposition process known per se, for example, by the vapour deposition of cadmium sulphide (CdS) or zinc sulphide (ZnS). By this process the piezo-electric device may be formed directly at the desired position on the device thereby eliminating the bonding operation normally required when a piezo-electric crystal is used.

We claim:

1. An electro-mechanical resonator comprising a vibrator beam of pre-determined resonant vibration frequency in a flexual mode, a U member of substantial stiflness relative to that of the beam and across the limb ends of which said beam is bridged and to which said beam is attached at its ends forming a beam-U member structure, electro-magnetic means coupled to said beam between said ends for causing said beam to vibrate resonantly in a flexual mode, and at least one piezo-electric device adapted to provide electrical output at said resonant vibration frequency when the beam vibrates in a flexual mode, said device being mounted or deposited on the beam-U member structure at a-location at which vibration occurs when the beam is vibrating in said flexual mode and having an electrode deposited on or otherwise bonded to said beam-U member structure at said location.

2. A resonator as claimed in claim 1 wherein said piezo-electric device providing the electrical output is mounted or deposited on the beam near one end thereof.

3. A resonator as claimed in claim 1 wherein said piezo-electric device providing the electric output is mounted or deposited on the U member ata location which is a vibration node.

4. A resonator as claimed in claim 1 wherein the U member is a rectangular U consisting of two parallel limbs and a cross piece at right angles to the same and the piezo-electric device providing the electrical output is mounted or deposited near one end of said cross piece.

5. A resonator as claimed in claim 1 wherein said electro-magnetic means is an electro-magnet upstanding from the middle of the U member and extending toward the middle of the beam with its free end spaced from said beam.

6. A resonator as claimed in claim 1 wherein said electro-magnetic means is an electro-magnet upstanding from the middle of the U member and having its free end adjacent but spaced from the beam and said piezoelectric device is mounted on said U member near one end of the part thereof Which joins the limbs thereof.

7. A resonator as claimed in claim 1 including mounting means carrying the U member at vibration nodes thereon.

8. A resonator as claimed in claim 1 wherein the piezo-electric device is deposited by a vapour deposition process.

9. A resonator as claimed in claim 8 wherein the said device is formed by vapour deposition of cadmium sulphide (CdS).

10. A resonator as claimed in claim 8 wherein the said device is formed by vapour deposition of zinc sulphide (ZnS).

References Cited UNITED STATES PATENTS Donley 310-82 X Roberts 333-71 X Burns 310-82 X Bercovitz 333-71 Mason 333-71 Blurn 310-83 X Kawakami 333-71 Yoshihiro 33,3-71 X Brouillette et al 3l0-8.2 Jacobsen 333-72 Hart 333-72 Hutson 3108.0 Hutson 310-80 Yando 333-72 Potter 2 333-72 Severs 310-82 X Peek 333-72 Mason 333-72 Von Ardenne 3l0-8.0 X

M. BUDD, Assistant Examiner US. 01. X.R 

